• The gap between official and actual vehicle efficiency and CO2 emissions is analyzed.

• An own elaborated database that provides real information on more than 650 cars is used.

• Econometric approach was applied for the period 2010-2017.

• Results show the biggest gap relate both to hybrid vehicles and to the biggest selling vehicles.

• Moreover, the average deviation rate decreased following ‘Dieselgate’

Literature explaining the gap between official and actual vehicle efficiency and CO2 emissions focuses on descriptive analysis to calculate this gap without examining causality. In this paper, we explore this discrepancy in detail by drawing on a database from Emissions Analytics Ltd. that provides on-road emissions measurement on more than 650 vehicles in the period 2010-2017.

The data reveal concerning results: firstly, the gap in data relates both to hybrid vehicles (that are supposedly ‘more fuel-efficient’) and to the biggest selling vehicles (medium-sized cars). Secondly, the average deviation rate increased prior to 2015, but decreased following ‘Dieselgate’.

The Volkswagen scandal threw light on the discretionary behaviour of manufacturers on this question and highlighted how weak the official tests are: and this in turn points to a regulatory and compliance problem. In other words, the interpretation of the results suggests that after several years of adaptation to the protocol and the corresponding test (but not translated in real consumption), manufacturers have taken measures to reduce the divergence in real terms after the scandal.

The General Court of the European Union overturned the emissions compliance levels under the new Real Driving Emissions (RDE) regulation in a verdict announced on 13 December. On the surface of it, this may look like a victory for cities wanting to be tougher on emissions of nitrogen oxides (NOx) from passenger cars and vans. In reality, it has the potential to further complicate the Euro 6 regulatory stage and thereby create the unintended consequence of making it even less usable for urban vehicle policy.

While the European Commission did not seek to increase the headline 80 mg/km emissions limit for diesel vehicles, which must still be adhered to in the laboratory test, they granted “Conformity Factors” that in effect did increase the limits for the harder, on-road part of the test. As a result, all diesels vehicles only had to meet 168 mg/km by September 2019, falling to 114 mg/km by January 2021. This was in practice a large increase in the permissible emissions limit.

The Court verdict suggests that vehicles already certified under Real Driving Emissions (that starts with the stage known as “Euro 6d-temp”) will remain compliant, as will vehicles certified for up to 14 further months in the future, depending on whether the Commission appeals and the speed with which replacement legislation is brought forward.

But what is this likely to mean in practice?

Let us assume that there is no appeal and no new legislation, meaning vehicles must meet 80 mg/km on the RDE test. Taking a sample of 30 RDE-certified diesel cars tested by Emissions Analytics on its independent EQUA Index test, we conclude that up to 90% of the vehicles would still be compliant. Although the test does not include cold start in its ratings, overall it remains a good proxy for RDE compliance. Furthermore, the average NOx on a combined cycle of these still-compliant vehicles is just 50 mg/km, well below the certification standard. Of the remaining 10%, they all come from the same OEM, Honda, which would in this scenario need to make changes.

This suggests the impact would be low. However, RDE does not become mandatory for all vehicles sold until September 2019. Therefore, it is likely that this conclusion is flattered due to a self-selecting sample of the best performing cars. If we look at the wider population of pre-RDE Euro 6 diesel cars, we may have a proxy for the challenge to each manufacturer of no Conformity Factors.

The table below shows the proportion of pre-RDE Euro 6 diesels from a range of manufacturer groups that meet various emissions levels, on our real-world scale. For example, 33% of all BMW/Mini Euro 6 diesel vehicles tested by Emissions Analytics achieve a rating of ‘A’

The ‘A’ rating is equivalent to 80 mg/km; ‘A’ to ‘C’ is equivalent to 180 mg/km (the Euro 5 level, close to the 2.1 Conformity Factor level); ‘A’ to ‘D’ is equivalent to 250 mg/km (Euro 4).

Approximately, the Conformity Factors allowed ratings of up to ‘C’, but with this annulled, ‘A’ ratings would need to be achieved. The table lists the five manufacturer groups that would be best placed to meet this requirement – assuming their pre-RDE performance is a proxy indicator of how close they are to achieving 80 mg/km across their whole range. VW Group, therefore, is best placed overall; Jaguar Land Rover has made the most rapid advances in the last year.

The final column of the table indicates the proportion that meet ‘A’ to ‘D’. This is relevant to the 270 mg/km maximum limit proposed in an agreement between the German government and cities, to be judged in real-world conditions. The ‘D’ rating is a near equivalent to this level. While the 270 mg/km limit is currently only proposed to apply to Euro 4 and 5 diesels, in time it could be extended to Euro 6 vehicles. If this were to happen, the table shows what proportion of these manufacturers’ vehicles may be restricted from the 14 German cities cited. Therefore, VW, BMW and JLR would be the least affected of all. In addition, this is before the benefit of the fixes and retrofits that have been, or may be, actioned, which would further reduce the restrictions. In contrast, there are manufacturers that could face having all their Euro 6 diesels restricted.

Overall, this may give reassurance to some, but there is a wider risk. The annulment of these Conformity Factors further confuses what “Euro 6” means as a label of performance. RDE was meant to be a discontinuity with the past, failed regulation. It had two levels already – two Conformity Factors – but the effect of the Court judgement could lead a change in the gradations or, potentially, more gradations. The Euro 6 label has limited informational content already, but the effect may be to cloud what RDE means, causing further consumer confusion, which would not be to the advantage of the car market.

The Court judgement is witness to the growing power of cities in determining vehicle emissions policy, even if they sometimes demonstrate an unresolved tension between whether air quality improvement or greenhouse gas emissions reduction is the higher priority. What the Court judgement may do is bring into starker relief the difference between the best and worst performing vehicles, which would pave the way for more efficient solutions to the urban air quality challenge.

This webinar seeks to demonstrate real-world emissions performance on the most recently certified passenger cars in Europe, and to examine what that reveals about the automakers approach to emissions reduction and compliance.

Thank you all who joined the webinar with Nick Molden, Emissions Analytics’ Founder and CEO on 15th October 2018.

Much rides on the new Real Driving Emissions regulation working: the credibility of the auto industry and regulators, as well as consumer confidence in the internal combustion engine. Specifically, the webinar will cover:

Aggressive driving on average increases pollutant emissions by 35% in rural driving and by around five times on the motorway, according to testing of the latest passenger cars by Emissions Analytics on its EQUA Index programme. Even higher “hotspots” have also been identified, where emissions at high speed can peak at more than ten times typical levels of nitrogen oxides (NOx) – the pollutant gas that was at the centre of the #dieselgate scandal.

The need to identify hotspots is becoming vital with the new Real Driving Emissions (RDE) regulations, which is a much tougher regulation of driving in normal conditions. The consequence of this will be that a greater proportion of total emissions may be concentrated in a small number of more unusual or extreme events. Unless those are well understood, the effect of the new regulations may be blunted.

The in-use surveillance requirements set out in the fourth package of RDE are aimed at monitoring vehicle compliance in all normal driving conditions, not just the cycle on which the vehicle was certified. Broadly, a vehicle should pass any RDE test within its useful life, whenever and whoever conducts the test. This is both a significant challenge for manufacturers, and brings with it risk as it is impossible a priori to guarantee compliance on all possible RDE tests.

To help quantify this risk, Emissions Analytics is launching a new evaluation programme that will quantify the risk of excessive emissions for each vehicle tested. Currently, EQUA Index ratings (www.equaindex.com) are published to allow the performance of different vehicles to be compared on a standard, normal cycle. This new programme leaves that rating unchanged, but puts the vehicle through an extended test designed to measure performance in more extreme and unusual driving conditions. The variance between that, the standard EQUA Index and the regulated level will yield a rating for the risk of exceeding the regulated level.

Considering eight diesel cars certified to the new RDE standard (Euro 6d-temp), the effect of driving at speeds up to 160 kph can be shown in the chart below.

In all cases the NOx emissions on the standard cycle – with maximum speeds up to 110 kph – are within the regulated limit of 80 mg/km plus 2.1 times conformity factor, even though certification does not apply this to the motorway section separately. In fact, many of the vehicles are comfortably below this limit. Allowing the maximum speed to rise to 160 kph shows significant proportionate increases on all but one vehicle, with the average percentage increase across all eight vehicles being 552%. All but two of the vehicles remain below the limit despite the increases; however, the worst two vehicles emitted around 650 mg/km.

For reference, under the RDE regulation, the vehicle’s velocity can be driven between 145 and 160 kph for up to 3% of the total motorway driving time. The risk of compliance therefore comes from a vehicle that has a significant emissions uplift at 160 kph and is relatively close to the limit at more moderate speeds.

Under cold start, vehicles 7 and 8 also showed an average increase in emissions of 160% compared to an average of 110% across the other vehicles.

Putting this data together with performance in other parts of the test cycle, it is possible to derive ratings of the risk of excessive emissions on RDE and on RDE-like cycles but with more relaxed boundaries, as shown in the table below.

It is important to note that a red rating does not necessarily imply non-compliance but, rather, it identifies elevated risk of non-compliance using results from the Emissions Analytics’ test, which runs a cycle similar to RDE but that is not strictly compliant.

Considering Euro 6 diesels, whether RDE or prior, the effect of cold start is that NOx emissions are 2.8 times higher on average during the cold start phase compared to the whole warm start cycle. During regeneration of the diesel particulate filter NOx emissions are on average 3.3 times higher than in mixed driving with no regeneration. Therefore, the frequency and geographical location of these events can be critical to the overall real-world vehicle emissions.

These results are important for cities, manufacturers and regulators. For cities, it is vital to know that the latest vehicles do not have emissions hotspots that could undermine their air quality targets. For manufacturers, facing third-party RDE testing to check compliance, it is important to quantify the risk of high emissions being found in unusual driving conditions, where every scenario cannot practically be tested. For regulators, it is important that RDE is seen to function well in order to draw a line under the failed regulation of the past.

Emissions Analytics will continue to test a wide range of the latest vehicles to publish comparable ratings between vehicles, but now with the added quantification of the risk of elevated emissions around the boundaries of normal driving.

The first diesel vehicle that met the regulated Euro 6 limit for nitrogen oxides (NOx) on our real-world EQUA Index (www.equaindex.com) test using a Portable Emissions Measurement System (PEMS) was in May 2013. Of the vehicles we tested in that year, the cleanest 10% of diesels emitted 265 mg/km and the dirtiest 10% emitted 1777 mg/km – a ratio of 7 to 1. In 2017, the cleanest 10% achieved an impressive 32 mg/km, but the dirtiest 10% were 1020 mg/km, a ratio of 32 to 1.

On average, progress has certainly been made, with average diesel NOx emissions having fallen from 812 mg/km to 364 mg/km from Euro 5 to Euro 6, or a 55% reduction, driven by the prospect of the new Real Driving Emissions (RDE) regulations together with the aftermath of dieselgate. The very worst vehicles have now disappeared from the new car market. It is also true that in around 10 years’ time, the majority of diesels on the road are likely to be of the cleaner variety, through natural turnover of the fleet.

We have now tested six of the latest RDE-compliant diesel vehicles, also known as ‘Euro 6d-temp’. Their average NOx emissions were 48 mg/km, 40% below the regulated limit itself, and 71% below the effective limit once the Conformity Factor of 2.1 is taken into account. (As ever, it should be noted that while the EQUA Index test is broadly similar to an RDE test, it is not strictly compliant.) However, it should be noted that there are many cleaner diesels even before RDE, with 30 prior models achieving real-world emissions of 80 mg/km or less.

While this sounds like good news, the elongated transition to RDE, and growing spread from the best to the worst, are creating a growing policy and consumer choice problem in the meantime. A vehicle in the highest-emitting decile today will likely be a significant contributor to urban NO2 pollution. Yet, the cleanest diesels are getting close to the average NOx emissions from new gasoline vehicles, which is 36 mg/km. Without the contemporary data to show this, policy makers would be forgiven for simply banning all diesels from urban locations.

The lowest NOx emission recorded so far this year is the 2017 model year Mercedes CLS, with selective catalytic reduction after-treatment and type-approved for 6d-temp, which recorded 15 mg of NOx per km.

Scrappage schemes are controversial. In a 2011 academic paper* reviewing 26 studies assessing the outcomes of 18 scrappage schemes implemented around the world in 2008-11, the authors concluded that the emission effects of the schemes were ‘modest and occur within the short term.’ They also concluded that the cost-effectiveness of such schemes ‘is often quite poor.’

The reality of the 2008-9 scrappage schemes, however, was that governments in Europe, the US and Japan were tackling a liquidity gap by stimulating consumption and bolstering an ailing car industry. The mooted environmental benefits of accelerated fleet renewal were talked up by politicians but were not the main objective. This helps to explain why the efficiency of the schemes in mitigating Greenhouse Gas (GHG) emissions – CO2 dominating discussion at the time – was found to be poor value for the taxpayer and of marginal consequence to overall path reductions towards a low carbon economy.

Since 2008-9, the policy climate has changed significantly, with air quality emerging as a major concern. Several national governments have been sued by environmental groups for illegal levels of NOx in cities, and what was once a transport policy issue has become a matter of public health. With diesel bans looming in city centres, plus the advent of clean air zones, it’s no stunt to reimagine scrappage according to an emission-reduction imperative.

Data availability upon which to base an intelligent scrappage scheme has also improved. Since 2011 Emissions Analytics has compiled the world’s largest database (the EQUA Index) of standardised real-world emissions tests, of over 2000 cars. The EQUA Index test measures not just CO2 emissions but nitrogen dioxide (NO2) and carbon monoxide (CO), as well as fuel efficiency.

Reimagining scrappage in light of air quality first requires accepting that the now discredited New European Driving Cycle (NEDC) test regime has produced counterintuitive outcomes, to the point where some of the newest cars are by no means the cleanest. This means that a poorly designed scrappage scheme could produce a worse outcome, measured in air quality, than doing nothing at all.

The EQUA Index results show that:

Dirtiest Euro 6 diesels are 6-7 times worse than cleanest Euro 5
Dirtiest Euro 6 diesels are up to 3 times worse than cleanest Euro 3/4
Some 20-year-old cars are cleaner than some brand new cars

How one weights GHG emissions against NOx emissions is a matter of policy debate and public acceptance, but a plausible objective would be Pareto optimality, or the idea that addressing air quality should not be at the expense of GHG such as CO2. While research points towards life cycle or ‘well to wheel’ analysis of CO2 emissions, this is difficult to measure and for the sake of a near-term scrappage scheme probably lies in the future.

Beyond the well-observed conflict between efficiency, where diesel scores well compared to gasoline, and the point-of-use emissions, where gasoline is consistently cleaner with respect to certain emissions such as nitrogen dioxide, the point to make is that it is possible to target and scrap dirty cars without increasing CO2.

The EQUA Index test recently revealed that 11 Euro 6 diesel cars from four manufacturers merited an A+ rating, equivalent to 0.06 g/km NOx in the EQUA Index test, which compares with a limit 180% higher (0.168 g/km NOx) for the new RDE requirement that will prevail until January 2021. Such cars can be said to be genuinely clean by today’s standards, but were greatly outnumbered by highly polluting diesel models, 38 of which scored F, G or H, meaning that they exceeded the Euro 6 limit in respect of NOx by 6-15 times. By comparison, 105 gasoline and hybrid Euro 6 cars achieved the A+ rating, and only one gasoline model fell into the D category, the rest being C or higher.

In the graph above, the arrow labelled ‘Bad Trade’ travels from a Euro 4, 1.9-litre diesel Skoda Octavia, model year 2009, that scored E in the EQUA Index test, towards a Euro 6, 1.6 litre diesel Nissan Qashqai, model year 2016, that scored H in the EQUA Index test. The Skoda is cleaner than the Nissan in real-world testing, yet a scrappage scheme designed around vehicle age alone could result in the cleaner Skoda being scrapped for purchase of an equivalent, dirtier Nissan Qashqai. This would result in tailpipe emissions higher by a factor of more than five and therefore worse air quality. It would be a waste of tax payer money.

Conversely the ‘Good Trade’ arrow highlights that dirty older (and conceivably newer) vehicles could be scrapped for genuinely cleaner new vehicles. Currently, 87% of Euro 6 diesel cars are over the Euro 6 limit, as are all Euro 5 cars. If all these cars were replaced by Euro 6 cars actually performing to the Euro 6 diesel standard in real-world conditions, the net tailpipe emissions improvement measured in NOx would be around 88% from the Euro 5/6 fleet. If real-world performance were only brought down to the Euro 5 diesel standard, the reduction in emissions would be 74%. Even if performance were reduced to just Euro 3 levels, there would still be a 38% reduction.

Put another way, as most Euro 5 and 6 diesels emit over the limit, a large number of vehicles need to be “fixed” to address the air quality problem. This inherently makes any scrappage scheme highly costly. Therefore, a more stratified approach may be optimal, as shown in the chart above. Emerging passenger car retrofit technology may deliver a 25% or more reduction in NOx; those vehicles with moderately high emissions could be tackled in that way. A scrappage scheme could then be targeted on the dirtiest diesels (perhaps worse than the Euro 3 level in real-world).

A further potential ‘Bad Trade’ may be switching from a diesel car to a non-hybridised gasoline car. The same vehicles viewed through the lens of CO2 emissions show that on average, gasoline vehicles are, like-for-like, a ratings class worse than diesels for absolute CO2 emissions. They also exhibit a greater disparity between NEDC measurements and actual emissions. For example, the 2017 Audi Q2 diesel, achieves A+ for air quality and C2 for CO2. The same power output gasoline equivalent model from the same year also achieves A+ for air quality, but a lower D4 for CO2. C means 150-175g/km CO2; D means 175-200 g/km, but the numbers 2 and 4 respectively show that the gasoline engine model is at greater variance from the officially claimed figures. This example highlights an element of the trade-off involved if tackling air quality results in a more gasoline-dominant fleet.

In summary, the right scrappage and retrofit schemes could incentivise consumers towards vehicles that are genuinely clean and genuinely efficient, taking into account not just NOx emissions, but also CO2 and particulates. This would contrast with the UK’s 2009 scheme that merely required customers to scrap their old car for any new vehicle. A stratified and discriminating scheme would require a more focussed replacement, resulting in better results both for air quality and climate change.

Pollution is a major contributor to chronic human sickness, not just environmental damage, according to the 2017 annual report of England’s Chief Medical Officer, Professor Dame Sally Davies, released on 2 March 2018. The report made 22 policy recommendations, many of which related to monitoring and ameliorating pollutant emissions. Emissions Analytics is pleased to have had its EQUA Index real-world emissions rating system (www.equaindex.com) cited in the report.

Electric vehicles have no tailpipe emissions – obviously. They do have indirect emissions from upstream manufacturing, and in-use emissions from tyre and brake wear, but it is range and efficiency which are of direct practical importance to owners.

As range anxiety diminishes with larger batteries, the relative efficiency of EVs will become more important in choosing the best vehicle. More challenging, for car buyers, is weighing the advantages and disadvantages of EVs against traditional powertrains, as they decide whether to switch. As a result, Emissions Analytics has extended its EQUA Index programme to test these new powertrains in a comparable way, with our partners Motor Trend (www.motortrend.com/real-mpg).

Recently, we put the Tesla 3 through the standard EQUA Real Mpg fuel economy test in the California, which is the same test we put internal combustion engine vehicles and hybrids through. The Tesla performed well, achieving efficiency of 3.1 miles per kWh. While there is no ideal way to convert this to a miles-per-gallon equivalent, if the kWh are converted to gallons based on relative energy content, this makes 103.7 (US) mpg, 124.5 (Imperial) mpg or 2.27 litres per 100 km.

This was a good performance, but not best-in-class. The 2017 Chevrolet Bolt we tested achieved 122.2 US mpg and the 2017 Hyundai Ioniq EV reached an impressive 151.8 mpg.

This is significant because it shows that the electric vehicle market is not just dominated by one player, but there are now a number of rival vehicles, with varying performance – information the consumer should have when making a purchase decision. Emissions Analytics’ EQUA Real Mpg data for the US market can be found at usa.equaindex.com, and the equivalent European data at www.equaindex.com.

At the same time as these developments, behind the scenes, Emissions Analytics has initiated a process to formalise its methods and evolve it to be relevant for testing the latest vehicles, including European diesels under Real Driving Emissions and EVs. In November, the inaugural workshop of this “CEN” process was held in Brussels. CEN, or Comité Européen de Normalisation, is a framework for standardisation of products and techniques across the European Union. After a period of open scrutiny and discussion, the testing methodology could become an official voluntary standard, for any organisation to use.

Emissions Analytics is undertaking this as part of its commitment to the recently-launched not-for-profit global alliance called “Allow Independent Road-testing” or AIR (www.allowair.org). As part of this, we want to open up our methodology to third parties to conduct consistent tests, in order to grow the global database of comparable results.

AIR is a separate entity from Emissions Analytics and structured as an alliance allowing like-minded organisations to sign up to the principles of independent testing and labelling. Any organisation interested in finding out more about the objectives and opportunities for membership, should contact Massimo Fedeli at mdefeli@allowair.org.

The link between these two recent events is that the ever-growing complexity of car choices needs an accurate, fair, trustworthy standard for measuring efficiency and emissions. Consumer trust must be rebuilt and cities need good tools to meet the air quality goals

The relevance of official fuel economy figures is still a perennial topic across the automotive industry, and one of the hardest to answer directly takes into account the human factor. Some claim that the official figures are in fact perfectly valid, if only the average driver wasn’t so lead-footed. But is there any truth in this train of thought?

Thinking laterally, one way to assess this question is to consider a scenario where the UK parc features a high penetration of connected and autonomous vehicles (CAVs). By taking the driver out of the loop an autonomous vehicle could deliver a more “responsible” and consistent driving style. This would in turn consume less fuel and so deliver significant reductions in nitrogen oxide (NOx) and carbon dioxide (CO2) emissions, then perhaps the driving style hypothesis is true.

To look at this very question Emissions Analytics and Imperial College London collaborated on the “Optimised Vehicle Autonomy for Ride and Emissions” feasibility project, supported by the Centre for Connected and Autonomous Vehicles and Innovate UK.

The research methodology started with Emissions Analytics performing on-road PEMS tests of 21 vehicles to its standard EQUA Index protocol across the UK and Germany. This provided second-by-second driving characteristics and the simultaneous emissions, allowing an emissions map for each car to be created.

These were then integrated with VISSIM traffic simulation software to model the effects of traffic flow and driving behaviour on the emissions. The simulation covered three main factors: vehicle driving strategy, communication between vehicles and infrastructure, and the penetration of CAVs. This included scenarios were a CAV followed a normal vehicle, a non-CAV followed a CAV, and CAV following another CAV.

We also factored in the ability for CAVs to communicate with traffic lights allowing them to approach a junction knowing when they were going to change, and so avoid coming to a complete stop.

In essence, the virtual CAVs were configured to mimic a more cautious and gentle driver with better anticipation of the road ahead and smoother style of driving. The benefit of the approach was we could model a world of 100% CAV penetration, but also the transition to that point via a mixed fleet of CAVs and non-CAVs.

The headline results are that, with 100% CAV penetration with congested traffic, NOX and CO2 could be reduced by approximately 20%. The preliminary analysis assumes a simplified scenario with only diesel cars, so the results are not yet reflective of the current UK fleet. It is plausible that petrol vehicles would show a similar CO2 reduction, while NOX emissions are typically low NOX whatever. Overall, the results show that smoother traffic flow could have significant benefits in urban areas.

This suggests that optimised driving styles can deliver lower vehicle emissions. However, according to our EQUA Index (www.equaindex.com), on average diesel NOx emissions are approximately five times the regulated limit on average (399 mg/km), and CO2 emissions are 40% above official values (based on the New European Driving Cycle). This means that even if driven more responsibly the average vehicle will only reduce its NOx exceedances to about a factor of four, and the CO2 exceedances to 30%. While this is an improvement, and underlines the benefits of driver training initiatives, it strongly suggests that the driver is not responsible for the majority of the emissions exceedances observed.

The chart below shows the reduction in NOx, as the penetration of CAVs increases from 0% to 100%. The benefits are seen is a broadly linear way as penetration increases, although the majority of NOx emissions still come from non-CAV vehicles even when CAV penetration passes 60%.

In summary, even if the driver and all associated “bad” driving habits are eliminated, the current fleet of vehicles will still well exceed official CO2 values, and diesel vehicles will exceed the NOX limits. Modifying driver behaviour can offer a valuable mitigation to these exceedances, but real-world emissions are still primarily determined by vehicle selection.

Emissions Analytics’ EQUA Index (www.equaindex.com) has been revealing the cleanest and most efficient cars since it launched in 2016. Since then, technology and vehicle performance have improved to a point that a new class of even cleaner vehicles can now be revealed.

On 17 October, the Mayor of London, in collaboration with Emissions Analytics, launched an online Cleaner Vehicle Checker (www.london.gov.uk/cleaner-vehicle-checker) with a new EQUA Aq A+ rating for those vehicles with the very lowest emissions of nitrogen oxides (NOx). This identifies 105 Euro 6 petrols meeting this more stretching standard and 11 diesel engines from four manufacturers.

To achieve the A+ rating a vehicle must emit no more than 0.060 grams per kilometre of NOx across the real, on-road EQUA Index test, made up of equal proportions of urban, rural and motorway driving. This is 25% more stringent than the A rating of no more than 0.080 grams per kilometre. It is also significantly tougher than emissions required under the new, official Real Driving Emissions (RDE) regulation, under which 0.168 g/km (180% higher) are allowed until 2021.

A recent study, conducted by Imperial College London on behalf of the campaign group Allow Independent Road-testing (AIR, www.allowair.org), concluded that RDE and the EQUA Index tests are broadly similar in how demanding they are on the vehicle.

The new A+ rating illustrates dramatically the challenge that diesel engines face, but also shows that they shouldn’t be automatically consigned to the history books. Not that many years ago, governments encouraged car buyers to opt for diesel cars in order to reduce carbon dioxide (CO2) emissions. However, the NOx emissions were much higher (often five times higher, and more than the petrol cars they replaced), and even the CO2 emissions were higher than the official figures suggested (often 40% or more adrift).

With the progress in standard “full” hybrid technology – ignoring for a moment plug-in hybrids – the newest models now delivers Mpg comparable with diesels. This in-turn means that on average CO2 as well as NOx emissions are lower too – the best of both worlds.

Does this spell the end for diesel passenger cars? Maybe. The loss of confidence in the automotive industry combined with the political narrative, nationally and locally, may be hard to correct. Consider the new Volkswagen Passat 1.6 litre diesel, with an EQUA Aq rating of A+, an EQUA CO2 rating of B and EQUA Mpg of 52.6 mpg. Compared to the latest Hyundai Ioniq petrol hybrid with EQUA Aq of A+, EQUA CO2 of A and EQUA Mpg of 58.1 mpg. The diesel has impressively low NOx emissions, but the hybrid is beating it on fuel economy and CO2 emissions.

So, the battle between diesels and hybrids is on. Can diesels shrug off the legacy of those Euro 6 diesels still on the market with EQUA Aq H ratings (12 or more times the regulatory NOx limit)? Does the current performance of hybrids obviate the need for alternative powertrains to meet urban air quality goals? Whatever happens, the EQUA Index will be tracking through its independent, real-world test programme.

The EQUA Index data can be accessed for free at www.equaindex.com. There are four ratings for each vehicle: EQUA Aq rating for NOx; EQUA CO2 for carbon dioxide; EQUA CO for carbon monoxide; and EQUA Mpg for fuel economy. In publishing the EQUA Aq A+ ratings, the boundaries for categories B to H have not been changed.